@Courtney487 The best way to deal with it is to try and avoid any conflict. You could reach out to them and explain that they must have omitted to pay the pet fee in ‘error’ and ask them to pay it through the resolution centre. That way you are letting them know you accept that it may well have been a genuine mistake which they can now rectify. If they ignore you and don’t pay it then you can ask for it after they have left via the resolution centre along with any additional cleaning fees and damage if applicable. However, they may claim it is either a Service or ESA in which case, and depending upon where you are.. the following applies.
https://www.airbnb.co.uk/help/article/1869/accessibility-policy